O(n) single DFS solution for Longest Univalue Path problem (similar to Diameter of a tree)

Author: kaidul

Diff for: source-code/Longest_Univalue_Path.cpp

@@ -7,6 +7,48 @@
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
+// with single DFS(like diameter of a tree)
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+    int longestUnivaluePath(TreeNode* root, int& longestPath) {
+        if(!root) return 0;
+        int leftDepth = longestUnivaluePath(root->left, longestPath);
+        int rightDepth = longestUnivaluePath(root->right, longestPath);
+        
+        if(root->left and root->left->val == root->val) {
+            leftDepth++;
+        } else {
+            leftDepth = 0;
+        }
+        
+        if(root->right and root->right->val == root->val) {
+            rightDepth++;
+        } else {
+            rightDepth = 0;
+        }
+        
+        longestPath = max(longestPath, leftDepth + rightDepth);
+        
+        return max(leftDepth, rightDepth);
+    }
+public:
+    int longestUnivaluePath(TreeNode* root) {
+        int longestPath = 0;
+        longestUnivaluePath(root, longestPath);
+        
+        return longestPath;
+    }
+};
+
+// O(n^2)
 class Solution {
     int longestUnivaluePath(TreeNode* root, int val) {
         if(!root or root->val != val) return 0;